Multiple pole filters are desirable over single pole filters because the passband frequency typically remains linear over a wider frequency range and the attenuation at an upper frequency cutoff value is much sharper and distinct for multiple pole filters than single pole filters. Therefore, two pole filters are typically implemented. Also, two pole filters operate in a predictable, well-defined manner. A two pole filter may be implemented with an operational amplifier which receives a single input signal to be filtered.
Analog circuitry may be implemented with differential signals to improve accuracy and circuit performance. When differential signals exist, noise contributions and other errors which are coupled into the signals are cancelled by the balanced nature of the differential signals. When a single signal is used, noise contributions may create significant circuit performance degradation. Use of differential signals is particularly advantageous when low power supply voltages such as five volts or less are used. However, single signal outputs are typically required at the output of most circuits. Therefore, conversions from fully differential signals to a single signal must be made. Such conversions are typically implemented by an operational amplifier. An example of an operational amplifier circuit which converts fully differential inputs to a single ended output amplifier is taught in the copending patent application Ser. No. 07/003,173, now U.S. Pat. No. 4,720,686, and assigned to the assignee hereof. Others who have implemented a filter circuit having differential input signals have typically utilized at least two operational amplifier circuits to do so wherein one operational amplifier converts the differential signals to a single signal and the second operational amplifier performs a filtering function. Such circuits typically require a large amount of circuitry and power to perform both functions. Two pole filter circuits may also require the use of very large capacitor values and resistor values. When nonlinear resistors are implemented, significant harmonic distortion may be introduced into the signal path.